Silicon oxynitride is used in a variety of applications. The ability to vary the refractive index of silicon oxynitride over a wide range makes it an attractive material for optical applications. The refractive index of pure SiO.sub.2 is 1.46, and the refractive index of Si.sub.3 N.sub.4 is 2.1. Therefore, the refractive index of silica doped with nitrogen can be varied between 1.46 and 2.1. In addition. doping silica optical waveguides with nitrogen helps to prevent UV radiation damage to the waveguide which causes undesirable losses.
In optical waveguide applications, silicon oxynitride has been produced by plasma and nonplasma CVD processes, using silane and/or ammonia gases. For optical applications, however, use of ammonia is undesirable because ammonia contains hydrogen, and the resulting synthesized silicon oxynitride may contain a substantial proportion of hydrogen which significantly contributes to losses in the waveguide.
In addition, silane raw materials must be handled very carefully due to the violent reaction caused when air is introduced into a closed container of silane. Silane is typically used in producing thin films on semiconductor substrates, which requires the deposition of a film having good characteristics for semiconductor applications. In the manufacture of semiconductor thin films the properties of the film are more important than deposition rate. In the production of optical devices, however, large quantities of material must be produced quickly, and the deposition rates for producing optical devices such as optical waveguides are much faster than deposition rates for semiconductor thin films.
Silicon oxynitride may also be produced by the pyrolysis or hydrolysis of organometallic halides such as silicon tetrachloride. However, use of halides is not favored because the pyrolysis and hydrolysis of these materials produces chlorine or a very strong acid by-product, hydrochloric acid (HCl). Hydrochloric acid is detrimental not only to many deposition substrates and to reaction equipment but also is harmful to the environment.
Additionally, it is difficult to produce bulk silicon oxynitride and waveguide preforms using conventional outside vapor deposition (OVD) processes, which expose the deposited material to air. One difficulty encountered in forming silicon oxynitride using conventional OVD processes is that when processing occurs in a system open to air, oxygen atoms preferentially bond to silicon atoms over nitrogen atoms, forming silica instead of silicon oxynitride.
In a typical OVD process, a carrier gas is bubbled through a liquid organic silicon containing compound. The resulting vaporous compound is transported to a burner via a carrier gas, wherein the vaporous gas streams are combusted in a burner flame fueled with natural gas and oxygen. The presence of oxygen in conventional OVD processes converts the vaporous reactants to their respective oxides, exiting the burner orifice to form a stream of volatile gases and finely-divided spherical particles of soot that may be deposited onto a substrate forming a porous blank or preform of soot, for example, silica soot.
U.S. Pat. No. 5,152,819 to Blackwell et al., the disclosure of which is incorporated by reference, describes the use halide-free silicon containing compounds including octamethylcylotetrasilazane in an OVD process to produce high purity fused silica glass. Octamethylcyclotetrasilazane, [(CH3)2SiNH]4, hereinafter referred to as OMCTSZ, is a white solid at room temperature and has a boiling point of 225.degree. C. An OVD process described in U.S. Pat. No. 5,152,819, which used OMCTSZ as a feedstock for the process produced a pure silica soot with less than 0.01% nitrogen contained in the soot.
In view of the difficulties encountered in manufacturing silicon oxynitride, there is an explicit need for a method for manufacturing silicon oxynitride which avoids the aforementioned problems. Specifically, it would be desirable to provide a method for manufacturing silicon oxynitride which does not contain a substantial proportion of hydrogen. In addition, it would be desirable to provide a process which avoids the preferential bonding of oxygen atoms to silicon atoms, which results in the formation of pure silica.